1. Field of the Invention
The present invention relates to a porous composite oxide and a production method. More particularly, the present invention relates to a porous composite oxide, which has pores of adequate size even after firing at a high temperature, and a production method.
2. Description of the Related Art
Composite oxides refer to oxides of a form in which two or more types of metal oxides have been formed into a compound, and which ions of oxoacids are not present as structural units. One important application of composite oxides is in catalysts and catalyst supports, and a particularly important application is known to be as a catalyst for exhaust gas purification of internal combustion engines.
For example, a cerium-zirconium composite oxide has been proposed for use as the support of a three-way catalyst (see, for example, Japanese Unexamined Patent Publication No. 2002-220228). Three-way catalysts purify the exhaust gas of internal combustion engines by simultaneously performing oxidation of hydrocarbons (HC) and carbon monoxide (CO) and reduction of nitrogen oxides (NOx) contained in the exhaust gas, and cerium oxide is added to these three-way catalysts because it has an oxygen storage capacity (abbreviated as OSC) in which it is able store oxygen in an oxidizing atmosphere and release oxygen in a reducing atmosphere. However, when three-way catalysts, which contain a metal catalyst and cerium oxide, are used at high temperatures above 800° C., as the OSC decreases due to crystal growth of the cerium oxide, zirconium oxide is added to the cerium oxide to maintain a high OSC by suppressing the crystal growth of the cerium oxide, thereby resulting in the formation of a cerium-zirconium composite oxide.
In addition, the use of a composite oxide composed of two or more types of alumina, zirconia, titania, iron oxide, ceria and magnesia as a support for an NOx occlusion-reduction catalyst (see, for example, Japanese Unexamined Patent Publication No. 2001-170487) is common.
Typical known examples of production methods for composite oxide powder include a simultaneous powder firing method in which a powder of each metal oxide or its precursor such as a carbonate or a hydroxide are mixed and fired, a co-precipitation method in which alkali is added to an aqueous solution of a plurality of metal inorganic salts to neutralize the solution and form a colloidal dispersion of oxide or hydroxide, and an alkoxide method in which water is added to a plurality of metal alkoxides dissolved in an organic solvent to hydrolyze the metal alkoxides.
In the powder simultaneous firing method, there are limitations on the fineness of the powder, and firing at a high temperature is necessary to obtain a composite oxide from the powder. In the case of high-temperature firing, grain growth occurs and surface area decreases. It is actually therefore quite difficult to obtain a fine powder of a composite oxide having a high surface area and completely homogeneous at the atomic level. The co-precipitation method utilizes the neutralization precipitation reactions of a plurality of inorganic ions in an aqueous solution, and although the particle diameter of the resulting colloidal particles is fine, as the precipitation reaction of each inorganic ion is dependent on pH, individual colloidal particles tend to become particles of respective independent metal oxides or metal hydroxides, thereby preventing the formation of composite oxides that are uniformly mixed at the atomic level. Although conventional alkoxide methods have used the hydrolysis of a plurality of metal alkoxides in organic solvent, as the stability and hydrolysis reaction rate varies according to the type of metal alkoxide, there is an order of priority by which oxides are formed between metals, which also prevents the formation of composite oxides uniformly mixed at the atomic level.
In addition, although conventional methods employ pore control for their resulting composite oxides, they typically have a pore size distribution consisting mainly of small pores having a particle diameter of about 2-7 nm. In addition, as conventional composite oxides are composed of aggregates of primary particles and have small pores between the primary particles, in the case sintering has progressed at high temperatures, sintering progresses between these primary particles causing the crystals to atrophy overall and pore volume to decrease considerably.
In the invention described in the aforementioned Japanese Unexamined Patent Publication No. 2002-220228, although the pores of the support are 2-100 nm, this basically consists of merely increasing the pore volume between primary particles, and as a result, as sintering progresses between the primary particles, the crystals atrophy overall and pore volume decreases.